The present invention relates to the field of magnetic shielding, and more particularly, to a device for providing magnetic shielding for a magnet system used in nuclear spin tomography.
Picture-producing methods have been developed in the field of medical diagnostics wherein by calculation or measurement, integral resonance signals of the nuclei of a given body element to be examined (particularly that of a human body) are analyzed. From the spatial spin density and/or the relaxation time distribution obtained in this manner, an image similar to that of X-ray tomography may then be constructed. Various techniques of this manner are well known under the designation "Nuclear Spin Tomography" (nuclear magnetic resonance tomography) or "Zeugmatography".
Nuclear spin tomography requires a strong magnetic field. The magnetic field is typically generated by a so-called base field magnet which should be as homogeneous as possible in an imaging or examination range of a predetermined extent. The body to be examined is placed within the imaging or examination range along an axis generally coinciding with the axis of orientation of the magnetic base field. This base field is superimposed by stationary and/or pulsed gradient fields. Further, a separate antenna structure is required in order that a high-frequency alternating magnetic field can be excited for a short time. The antenna structure excites the individual atomic nuclei of the examined body into a precession motion. Frequently, the RF signals produced in this manner are received by the picture-producing means unless special measuring loops are provided to filter the signals.
The base field magnet may be designed as a system of, for example, six individual ring-shaped magnetic coils which are lined up one behind the other along a common axis in the direction of the desired field orientation of the magnetic base field. A frame structure, such as the one disclosed in European Patent No. 0 102 486 A1 comprising several frame parts which extend parallel to the common axis is used for mounting and/or spatially adjusting the individual magnetic coils. In order for the desired high homogeneity of the magnetic field to be achieved, fine geometric adjustments must be made to at least one of the individual coils. European Patent No. 0 102 486 A1, mentioned above, discloses adjusting and holding means which are particularly well suited for this purpose.
The prior art structure comprises a stationary frame made of a magnetically non-conductive material having two lower support rails for supporting the weight of the magnetic coils and an upper support rail for holding the magnetic coils. The frame parts extending parallel to the common axis are mechanically connected to each other through one or more of the individual magnetic coils to form a rigid structure. Special adjusting devices for the magnetic coils not firmly secured are located on the frame structure and allow the individual coils to be easily tipped from their perpendicular position relative to the orientation axis of the magnetic base field.
It is frequently desired in magnet systems of this type to provide a magnetic shield or a magnetic return. European Patent No. 0 067 933 B1 discloses a hollow, ferromagnetic return body enclosing an individual nuclear spin tomography coil. An advantageous modification of the hollow-cylindrical return body noted above is disclosed in European Patent No. 0 141 149 A1 wherein the return body is hexagonically formed of individual beam-like shielding elements. However, a problem with the prior art is that when such magnetic shielding is attempted to be used for an entire magnet system, the amount of shielding material necessary and the space requirements to implement the shielding are prohibitively large. Furthermore, access to the individual magnetic coils for effecting fine adjustments within the frame structure would be obstructed due to the magnetic shielding.